Got an overpayment notice, what now? - 19 weeks in

I received a letter saying I was overpaid about $150 on my claim. I do not usually post, so bear with me. The letter mentions an appeal window and a repayment plan. Who should I talk to first?

Thanks for writing this out. I went through something close to it 12 months ago, and it got easier once I kept a simple log of every call and form.